This invention relates generally to a method for treating a phyllosilicate mineral and to the product of such method and more particularly concerns a method for treating a phyllosilicate mineral by grinding a slurry of particles of the mineral in an organic liquid to thereby produce particles having a relatively greater specific surface area and also the product formed by such method.
It is well known that phyllosilicate minerals are useful in a variety of applications as fillers, pigments, adsorbents, absorbents, catalysts, inert diluents or carriers for materials adsorbed or otherwise immobilized on their surfaces. The specific surface area of a particular phyllosilicate mineral is an important property in determining the suitability of the mineral for such uses. In many applications, it has been found that the higher the specific surface area of a mineral, the more effective is the mineral.
When the mineral is to be used as a filler or pigment in a second material or as an adsorbent, absorbent or carrier for a second material, the bonding characteristics of the mineral to the second material are important factors in determining the suitability of the mineral in such use. The bonding characteristics in such cases are at least partly determined by the amount and character of the surface area of the mineral available for bonding to the second material. Similarly, surface area is a property which at least partly determines the adsorptive capacity of a mineral. For these and other reasons, it is generally considered to be highly desirable to devise means for increasing the surface area of mineral particles.
In the case of lamellar phyllosilicate mineral particles, a particularly effective method for increasing specific surface area is to fracture particles of the mineral and preferably to delaminate the mineral particles. In the present context, the term "fracture" shall include all forms of breaking the mineral particles, including delamination. Lamellar phyllosilicate minerals are crystalline silicate minerals which show a pronounced propensity toward cleavage perpendicular to one (and only one) crystalline axis. This arises as a result of much weaker bonding forces parallel to this particular crystallographic axis. When subjected to stress these lamellar materials tend to break along the planes where the bonding strength between adjacent planes is least.
Delamination is an extremely effective mechanism for increasing the surface area of lamellar materials because the relative amount of energy required to produce or expose additional surface area by delamination is much less than by other modes of fracture. In addition, the resulting delaminated materials are especially effective in applications where they are to be bonded to some other material. In this regard, due to the weak bonds between the individual plates of a lamellar material, lamellar materials are often relatively undesirable as reinforcing fillers in applications involving binding the lamellar material to a second material. Generally only the exterior surfaces of the lamellar material can be bonded to the second material, and, even if such bond is strong, the individual platelets are relatively weakly bonded to one another and may tend to slide over one another or otherwise separate from one another by interlaminar forces, when subjected to stress. Consequently, the laminations may tend to weaken the reinforcing capability of lamellar materials as fillers. Delamination eliminates the weak bonds between the platelets and thereby affords a delaminated material having potentially greater reinforcing characteristics over its laminated predecessor. Furthermore, the newly exposed surfaces of the resulting delaminated platelets may have especially good bonding characteristics due, for example, to active sites that may be formed thereon as a result of the energetic process associated with the particular delamination technique employed.
Numerous techniques have been proposed for increasing the surface area of silicate minerals. For example, Khodakov and Rebinder, Kolloidnyi Zhurnal Vol. 22, No. 3, pp. 365-375 (1960) disclose a process in which talc is ground in water, acetone or an alcohol such that the resulting ground particles have a relatively higher specific surface area and specifically illustrate increased specific surface areas as high as about 140 square meters per gram. The authors indicate that grinding the talc particles in the presence of less than about 5 percent of water, acetone or alcohol produces a significant increase in the specific surface area of the ground particles relative to the unground talc particles, but that grinding the talc particles in greater relative amounts of such liquids results in a decrease in the specific surface area of the ground particles relative to the unground talc particles. Though not mentioning talc, the authors indicate that particles of certain other materials can be ground once in a liquid and then ground again briefly in water in order to destroy any aggregates formed during the first grinding step and thereby to effect an apparent increase in the specific surface area.
Khodakov and Edelman, "Experimental Investigation of Spontaneous Dispersion in Molecularly Compact Solid Aggregates, "Kolloidnyi Zhurnal, Vol. 31, No. 5, pp. 771-776, September-October, 1969 cite the aforesaid article of Khodakov and Rebinder and disclose several treatments of minerals which result in an increase in surface area. The main thesis of the article is that grinding particles of a mineral may introduce defects and internal stresses into the particles and that, in a dispersed phase consisting of particles containing such defects and internal stresses, an increase in the degree of dispersion as a result of the spontaneous dispersion of such particles is feasible.
Using the examples of finely ground powders of quartz, calcite, corundum, and talc, consisting of molecularly compact aggregates of particles, the authors investigated the process of spontaneous dispersion. The effective specific surfaces of these powders (according to the BET method) was measured after a long-term holding in water. The study was made of highly dispersed powders of quartz, synthetic corundum, talc, and calcite, obtained by fine vibrational grinding either in air or in water, toluene, dichloroethane, acetone, ethyl alcohol, or amyl alcohol. The authors point out that in several experiments, the liquids amounted to a few percent of the weight of the powder. The authors do not indicate specific percentages when the liquid grinding additives amounted to a larger percent of the weight of the powder. Furthermore, no indication is given of a difference in the form of the ground particles obtained by grinding particles in non-polar organic liquids or polar liquids such as water.
Khodakov and Edelman disclose that, by grinding, particles of the powders can be obtained in the form of unique, very dense aggregates made up of primary particles whose internal surfaces are not accessible to measurement by any of the methods of dispersion analysis, including the BET adsorption method. The authors also indicate that the molecularly compact aggregates are individual continuous solids, but with defects.
After grinding, the powders were dried and then were held for a long period of time at room temperature, either in water or in heptane, acetone or dimethylformamide. The kinetics of spontaneous dispersion (decomposition) were evaluated from the change with time of the specific surface (S) of the dried powders, as measured by the BET method from the adsorption of nitrogen. Khodakov and Edelman stated that Khodakov and Rebinder in their aforesaid article previously established the presence in the ground particles of molecularly compact aggregates of particles by short-term grinding of the powders in water and illustrated that molecularly compact aggregates decompose with relative ease with short-term grinding in an aqueous medium.
Of the powders employed in the study of Khodakov and Edelman, only quartz and talc are silicate materials, and only talc is a phyllosilicate mineral. As indicated in their article, the authors noted that it followed from FIGS. 1 and 2 that the rate of growth of the specific surface depends substantially on the grinding conditions of the powders and that a particularly great effect of spontaneous dispersion is observed for talc "under the given conditions." The only indication in the article of the conditions for grinding talc appear in the descriptive note beneath FIG. 1, where it is stated that the talc is dry-ground and thereafter stored for a period of over two months in water. Under such conditions, the specific surface of the talc increased to a maximum of about 90 square meters per gram within about three weeks of storage in the water. The temperature of the water and the relative amounts of talc and water are not indicated.
In addition, there are other references to grinding phyllosilicate minerals in an organic liquid. For example, Rosenthal, U.S. Pat. No. 230,538 discloses that asbestos or amianthus is contacted with coal oil, benzene, benzole, or other equivalent hydrocarbons, which may be cold or heated or applied under both heat and pressure, whereby the fibers of the asbestos or amianthus are reduced to a condition which permits their disintegration manually or mechanically.
Davenport, U.S. Pat. No. 1,829,039 discloses a method for producing mica powder by triturating fragmentary sheets of mica in water or other suitable liquid by a rubbing action. Water is the only liquid specifically disclosed.
Thomson, U.S. Pat. No. 1,950,829 discloses a method of treating vermiculite to provide thin sheets of micaceous material which comprises crushing the vermiculite, soaking the crushed vermiculite in water for a long enough time to permit the water to penetrate between the layers of the micaceous material, removing the soaked material from excess or superficial water, heating the removed material to a temperature of about 1800.degree. F. The resulting thin sheets may be further ground in water or oil to provide fine, thin particles.
Heyman, U.S. Pat. No. 2,405,576 discloses a method for splitting sheets of mica by striking the edge of the mica with a jet of a liquid, thus splitting it. The patent includes a statement that, although methyl alcohol or distilled water is preferred as the liquid, other liquids may prove to be equally effective for the purpose. Such other liquids are not identified.
Jacobs et al., U.S. Pat. No. 3,313,492 discloses a method for grinding finely-divided talc in water or other liquid, using a dispersing agent, if necessary, to impart fluidity to the slip. Water is the only liquid specifically disclosed.
None of the aforesaid references disclose a method which produces a product whose specific surface area is, or can be increased by additional treatment to, greater than about 140 square meters per gram. Moreover, the water soaking treatments following the grinding operations in Khodakov and Rebinder and in Khodakov and Edelman require additional grinding or excessive soaking time, respectively.